In the operation of a lithium ion battery, the anode takes up lithium ions from the cathode when the battery is being charged and releases those ions back to the cathode during discharge. One important parameter of the anode material is its capacity for retaining lithium ions, since this will directly impact the amount of charge that a given battery system can retain. Another important parameter is reversibility—the number of times the material can take up and release lithium ions without degradation or significant loss of capacity. This parameter will directly influence the service life of the battery system.
Lithium ion battery systems generally employ a carbonaceous anode due to the fact that it has very high reversibility and is quite safe. One problem with carbon materials is that their lithium ion capacity is only moderately high, hence, relatively large amounts of anode material must be employed in a given battery system. Silicon is capable of alloying with relatively large amounts of lithium and has a number of advantages as an anode material for lithium ion batteries. Typical carbon based anodes have discharge capacities of approximately 372 mAh/g while silicon has a theoretical capacity of 4200 mAh/g. Silicon, however, undergoes a relatively large volume change when lithium is incorporated therein. This volume change is very disadvantageous in most battery systems since it can cause a loss of capacity, decrease cycle life, and cause mechanical damage to the battery structure. Silicon expands volumetrically by up to 400% on full lithium insertion (lithiation), and it can contract significantly on lithium extraction (delithiation), creating two critical challenges: (1) minimizing the mechanical degradation of silicon structure in electrode and (2) maintaining the stability of the solid electrolyte interface (SEI). Stress induced by large changes in the volume of silicon anodes causes cracking and pulverization. Studies have shown these to be the main reasons for rapid capacity loss.
Tendency for fracture and decrepitation could be reduced or avoided by reducing the silicon particle size to the nanometer range. Indeed, the strain in such silicon nanostructures can be relaxed easily without mechanical structure, because of their small size and the available surrounding flee space. There has been some success in addressing the silicon material stability issues by designing nanostructured silicon materials including nanowires, nanotubes, nanoporous films and silicon nanoparticle/carbon composites. Some such approaches are disclosed in U.S. Patent Application Publications 2007/0077490, 2007/0190413 and 2005/0282070; U.S. Pat. No. 7,316,792, and published PCT Application WO 2007/015910.
However, SEI stability at the interface between the silicon and the liquid electrolyte is another critical factor in achieving a long cycle life. Even though the silicon mechanical fracture issues and decrepitation are largely overcome by using nanostructures, the interface with the electrolyte is not static due to their repetitive volume expansion and contraction. This represents a significant challenge that has not been effectively addressed for materials undergoing large volume changes.
Electrolyte decomposition occurs on the low potential anode and forms a passivating SEI layer on the silicon surface during battery charging. The SEI layer is an electronic insulator, but a lithium-ion conductor, so the growth of the SEI layer continues unabated on a freshly formed silicon surface resulting from silicon volume changes during cycling. Nano-scale structure has shown to minimize the stress induced mechanical breakdown, but could not address the SEI growth issue. Silicon expands upon lithiation, and then contacts during delithiation. This repetitive process causes cracks to form at the silicon surface. Even with the use of nano-structure the previously formed SEI can be broken during delithiation due to shrinking. The re-exposed fresh silicon surface to the electrolyte causes additional SEI to form resulting in thickening of the SEI with each charge/discharge cycle.
The thickening of the SEI results in a degradation of battery performance through: (1) the consumption of electrolyte and lithium ions during continuous SEI formation; (2) the electrically insulating nature of the SEI weakening the electrical contact between the current collector and anode material; (3) the long lithium diffusion distance through the thick SEI; and (4) electrode material degradation caused by mechanical stress from the thickening of SEI. The formation of a stable SEI is critical for realizing a long cycle life in silicon anodes. This also holds true generally for other electrode materials subject to large volume changes.
Despite the various efforts, the researchers have not been able to successfully utilize silicon based materials to prepare high capacity anode structures for lithium ion batteries that manifest high cycle lives. As will be explained in detail herein below, the present invention provides a pulverization resistant electrode material that is capable of alloying with large amounts of lithium ions and also retains this ability through a large number of charge/discharge cycles. The electrode materials of the present invention allow for the manufacture of high capacity lithium ion batteries that have superior cycle life. These and other advantages of the present invention will be apparent from the drawings, description and discussion which follow.